9.6.2 System and Performance Audits
of upper-air instrumentation to verify their proper operation pose some
interesting challenges. While system audits can be performed using
traditional system checks and alignment and
orientation techniques, performance audits of some instruments require
unique, and sometimes expensive
procedures. In particular, unlike surface meteorological instrumentation, the upper-air systems cannot be challenged using
known inputs such as rates of rotation, orientation
directions, or temperature baths. Recommended techniques for both system and performance audits of the upper-air instruments
are described below. These techniques have been
categorized into system audit checks and performance audit procedures for
systems, radar wind profilers, sodars, and RASS.
audits of an upper-air station should include a complete review of the QAPP,
any monitoring plan for the station, and the station's standard operating
procedures. The system audit will
determine if the procedures identified in these plans are followed during
station operation. Deviations from the
plans should be noted and an assessment made as to what effect the deviation
may have on data quality. To ensure consistency in the system audits, a
checklist should be used. System audits should be conducted at the beginning
of the monitoring program and annually
Sounding Systems For radiosonde sounding systems, an entire launch cycle
should be observed to ensure that the site technician is following the
appropriate procedures. The cycle begins
with the arrival of the operator at the site and ends with completion of the
sounding and securing of the station. The following items should be checked:
station initialization procedures should be reviewed to ensure proper
initialization procedures should be reviewed to verify that the sonde
has been properly calibrated.
inflation should be checked to ensure an appropriate ascent rate.
and secure attachment of sonde to balloon should be verified.
of the radio theodolite antenna should be checked, using solar sitings
when possible. The antenna alignment should be maintained within ±2°.
vertical angle of the radio theodolite antenna should be checked and
should be within ±0.5°.
acquisition procedures should be reviewed and a sample of the acquired
data should be inspected.
archiving and backup procedures should be reviewed.
termination and system shutdown procedures should be reviewed.
maintenance procedures should be reviewed and their implementation
should be checked.
processing and validation procedures should be reviewed to ensure that
questionable data are appropriately flagged and that processing
algorithms do not excessively smooth the
from several representative launches should be reviewed for
reasonableness and consistency.
logbooks, checklists, and calibration forms should be reviewed for
completeness and content to assure
that the entries are commensurate with the expectations in the
procedures for the site. Remote Sensing Instrumentation A
routine check of the monitoring station should be performed to ensure
that the local technician is
following all standard operating procedures (SOPs). In addition, the following items should be checked:
antenna and controller interface cables should be inspected for proper
connection. If multi-axis antennas are used, this includes checking for
the proper connection between the
controller and individual antennas.
checks should be performed on the individual antennas, or phased-array antenna. The checks should be verified using
solar sitings when possible. The measured orientation
of the antennas should be compared with the system software settings.
The antenna alignment should be maintained within ±2°.
multi-axis antennas, the inclination angle, or zenith angle from the
vertical, should be verified against the software settings and the
manufacturer's recommendations. The measured
zenith angle should be within ±0.5°
of the software setting in the data system.
phased-array antennas, the array should be level within ±0.5°
of the horizontal.
multi-axis sodar systems, a separate distinct pulse, or pulse train in
the case of frequency-coded pulse systems, should be heard from each of
the antennas. In a frequency-coded pulse system there may be a sound
pattern that can be verified. The instrument
manual should be referenced to determined whether there is such a
sodar systems, general noise levels should be measured, in dBA, to
assess the ambient conditions and their potential influence on the
performance of the sodar.
vista table for the site (see Section 9.5) should be reviewed. If a
table is not available then one should be prepared.
electronic systems and data acquisition software should be checked to
ensure that the instruments are operating in the proper mode and that
the data being collected are those specified
by the SOPs.
logbooks, checklists, and calibration forms should be reviewed for
completeness and content to assure that the entries are commensurate
with the expectations in the procedures for the site.
site operator should be interviewed to determine his/her knowledge of
system operation, maintenance, and proficiency in the performance of
quality control checks.
antenna enclosures should be inspected for structural integrity that may
cause failures as well as for any signs of debris that may cause
drainage problems in the event of rain or snow.
maintenance procedures should be reviewed for adequacy and
time clocks on the data acquisition systems should be checked and
compared to a standard of ±2 minutes.
data processing procedures and the methods for processing the data from
sub-hourly to hourly intervals should be reviewed for appropriateness.
collected over a multi-day period (e.g., 2-3 days) should be reviewed
for reasonableness and consistency. The review should include vertical
consistency within given profiles and temporal consistency from period
to period. For radar wind profilers and sodar, special attention should
be given to the possibility of contamination of the data by passive or
active noise sources.
Audit and Comparison Procedures
audits should be conducted at the beginning of the monitoring program and
annually thereafter. A final audit should be conducted at the conclusion of
the monitoring program. An overview of
the recommended procedures for performance auditing is provided below.
Performance auditing of radiosonde sounding systems presents a unique challenge in that the instrument is used only
once and is rarely recovered. Therefore, a performance audit of a single
sonde provides little value in assessing overall system performance. The
recommended approach is to audit only the instruments that are used to
provide ground truth data for the
radiosondes prior to launch (thermometer, relative humidity sensor,
psychrometer, barometer, etc.). The
reference instruments used to audit the site instruments should be traceable to a known standard. Details on these audit
methods can be found in reference .
addition, a qualitative assessment of the direction and speed of balloon
travel should be made during an observed
launch for comparison with the computed wind measurements. An alternative
approach is to attach a second sonde package to the balloon, track it from
an independent ground station, and
compare the results of the two systems. An optical tracking system
is adequate for this type of comparison.
Sensing Instrumentation Methods for performance audits and data
comparisons of remote sensing instrumentation have been under development
for a number of years. Only recently has
interim guidance reference  been released to help standardize
performance audit methods. Even with the
release of that guidance, there are still a number of areas undergoing
development. Recommended procedures for performance audits and data
comparisons of remote sensors which are presented below typically
incorporate inter-comparison checks. If inter-comparison checks
are used, a quick review of the datasets should be performed before dismantling
the comparison system.
The performance audit is used to establish confidence in the ability of the
sodar to accurately measure winds. A performance audit of a system typically
introduces a known value into the sensor
and evaluates the sensor response. It may not be possible to perform this
type of audit for all types of sodar
instruments. In this case, a comparison between the sodar and another measurement system of known accuracy should be
performed to establish the reasonableness of the
sodar data. With any of the audit or comparison methods, the evaluation of
the data should be performed on a
component specific basis that corresponds to the sodar beam directions. Any of the following approaches may be considered in
the sodar performance evaluation.
Comparison with data from an adjacent tall tower. Using this
approach, conventional surface
meteorological measurements from sensors mounted on tall towers (at
elevations within the operating range of
the sodar) are compared with the sodar data. This method should
only be used if the tall tower is an existing part of a monitoring program
and its measurements are valid and
representative of the sodar location. At least 24 hours of data should
be compared. The tower data should be time averaged to correspond to the
sodar averaging interval and the
comparisons should be made on a component basis. This comparison
will provide an overall evaluation of the sodar performance as well as a means for detecting potential active and passive
with data from another sodar. This comparison uses two sodars operating
on different frequencies. The comparison sodar should be located in an
area that will allow it to collect
data that is representative of the site sodar measurements. At least 24
hours of data should be collected for
the comparison. If the measurement levels of the two sodars
differ, the comparison sodar data should be volume averaged to
correspond with the site sodar.
Additionally, the comparison sodar time averaging should correspond to the site sodar. As with the adjacent tall
tower, the comparison should be performed on a component
basis. This comparison will provide an overall evaluation of the sodar performance as well as a means for detecting
potential active and passive noise sources.
with radiosonde data. This comparison uses data obtained from a
radiosonde carried aloft by a free-flight, slow-rise balloon. The
balloon should be inflated so the ascent
rate is about 2 ms -1 . This will provide the appropriate resolution for
the comparison data, within the
boundary layer. The wind data should be volume averaged to correspond
with the sodar data and the comparisons should be made on a component as well as a total vector basis. The launch
times should be selected to avoid periods of changing
meteorological conditions. For example, evaluation of the comparison
data should recognize the potential
differences due to differences in both the spatial and temporal
resolution of the measurements (i.e., the instantaneous data collected
by the radiosonde as compared with
the time averaged data collected by the sodar). This comparison
will provide an overall evaluation of the sodar performance as well as a means for detecting potential active and
passive noise sources.
with tethersonde data. The tethersonde comparison is performed using
single or multi-sonde systems. Using this approach, a tethered balloon
is used to lift the sonde(s) to altitude(s) corresponding with the sodar
measurement levels. This method should collect data at one or more
layers appropriate to the program objectives. At a minimum,
data corresponding to the equivalent of five sodar averaging periods
should be collected at each altitude.
Multiple altitudes can be collected simultaneously using a multi-sonde
system with two or more sondes. The individual sonde readings should be processed into components that correspond to
the sodar beam directions and then time averaged
to correspond to the sodar averaging period. This comparison will
provide an overall evaluation of the
sodar performance as well as a means for detecting potential active
and passive noise sources.
with data from an anemometer kite. This measurement system is suitable
in relatively high wind speed conditions that would preclude the use of
a tethersonde. The kite anemometer
consists of a small sled type kite attached to a calibrated spring
gauge. Horizontal wind speeds are
determined from the pull of the kite on the spring gauge. The altitude
of the kite (i.e. the height of the measured wind) is determined from
the elevation angle and the distance
to the kite. The wind direction is determined by measuring the azimuth
angle to the kite. At a minimum, data corresponding to the equivalent of
five sodar averaging periods should
be collected at a level appropriate to the monitoring program
objectives. The wind speed and kite azimuth and elevation readings
should be taken every minute. The
individual readings should be processed into components that correspond
to the sodar beam directions and then time averaged to correspond to the sodar averaging period. This comparison will
provide an overall evaluation of the sodar performance
as well as a means for detecting potential active and passive noise
of a pulse transponding system. A pulse transponding system provides a
means of testing the sodar system processing electronics for accuracy
through the interpretation of simulated Doppler shifted signals at known
time intervals . This method can be considered
an audit rather than a comparison because it provides a signal input
equivalent to a known wind speed,
wind direction and altitude to test the response of a sodar system. At least three averaging periods of
transponder data should be collected with the sodar in its
normal operating mode. Depending on the sodar configuration, this method
along with an evaluation of the
internal consistency of the sodar data to identify potential passive
and active noise sources, may serve as the performance audit without the
need of further comparisons. In the
case of phased array sodars, an additional comparison is needed
to verify proper beam steering. This comparison may be performed using
any of the methods above. For this
check, three sodar averaging periods at a single level are sufficient.
It should be noted that current transponder technology is limited to
sodars with three beams.
Wind Profilers. At present, the performance of radar wind profilers can only
be evaluated by comparison to collocated or nearby upper-air measurements.
Various types of comparison instruments
can be used including tall towers, sodar, radiosonde sounding systems, and tethersondes. A tethersonde may be used, but
care should be taken to ensure that it does not interfere
with the radar operation. Since it is important to have confidence in the
reference instrument, an independent
verification of operation of the reference instrument should also be obtained.
If using a sodar or a radiosonde sounding system, the procedures outlined
above should be followed to ensure acceptable operation of the system. If
data from an adjacent tower are used,
then it is recommended that the quality of the tower-based data be
established. The comparison methods should follow those described for sodars
above. Where RASS acoustic sources may
interfere with the comparison sodar operation, care should be taken to
identify potentially contaminated data.
Like the radar wind profiler, the evaluation of a RASS relies on a
comparison to a reference instrument. The
recommended method is to use a radiosonde sounding system to measure
the variables needed to calculate virtual temperature (i.e., pressure,
temperature, and humidity). Sufficient
soundings should be made for comparisons during different times of the day to evaluate the performance of the system
under different meteorological conditions. Data collected
from the sonde should be volume averaged into intervals consistent with the
RASS averaging volumes, and the values
should be compared on a level-by-level and overall basis.
9. UPPER-AIR MONITORING
9.1.1 Upper-Air Meteorological Variables
9.1.2 Radiosonde Sounding System
9.1.3 Doppler Sodar
9.1.4 Radar Wind Profiler
9.2 Performance Characteristics
9.2.1 Definition of Performance Specifications
9.2.2 Performance Characteristics of Radiosonde Sounding Systems
9.2.3 Performance Characteristics of Remote Sensing Systems
9.3 Monitoring Objectives and Goals
9.3.1 Data Quality Objectives
9.4 Siting and Exposure
9.5 Installation and Acceptance Testing
9.6 Quality Assurance and Quality Control
9.6.1 Calibration Methods
9.6.2 System and Performance Audits
9.6.3 Standard Operating Procedures
9.6.4 Operational Checks and Preventive Maintenance
9.6.5 Corrective Action and Reporting
9.6.6 Common Problems Encountered in Upper-Air Data Collection
9.7 Data Processing and Management (DP&M)
9.7.1 Overview of Data Products
9.7.2 Steps in DP&M
9.7.3 Data Archiving
9.8 Recommendations for Upper-Air Data Collection